Microwave device



March 23, 1943. E, G UNDER MICROWAVE DEVICE Filed June 25, 1940 (255#$555 fatwa-.5155

IIII A ,e5/M207 'maentor Patented Mar. ,1943

UNITE-o STATES PATENT orrice MICROWAVE DEVICE Ernest G. Linder,Philadelphia, Pa., assignor to' Radio Corporation of America, acorporation of Delaware Application Julie 25,

1 Claim.

This invention relates to ultra high frequency devices, and moreparticularly to methods of and apparatus forgenerating, detectingand'amplifying microwave radio energy by means of resonant cavities. v

I am aware of devices in which a constant stream, of electrons is causedto flow through a resonant cavity in such a manner that the standingWaves therein accelera-te some electrons and decelerate others toproduce spaced groups of electrons. This system is known as velocitymodulation, and the total number of electrons leaving the cathode over agiven period of time is constant. It is the principal object of thisinvention, however, to provide means in a resonant cavity device wherebycontrol is effected by varying the electron emission itself. Otherobjects include the provision of an improved resonant cavity device; theprovision of an improved resonant cavity microwave detector; theprovision of a multielement resonant cavity discharge device; theprovision of a diode rectier in which interelectrode capacities do notdeleteriously affect the operation of the device: and the provision of aresonant cavity discharge device which may be operated as an amplifier`rectifier or oscillator.

The foregoing objects are accomplished by means of a device in which theemitting surface of an electron-producing cathode is mounted in aresonant cavity so that it is directly affected by the standing waveswhich are set up therein. The cavity electrode itself may function as ananode electrode. or a separate anode may be provided. In an alternativeembodiment of this invention, a grid electrode is employed to aid incontrolling the electron stream. In another embodiment, two resonantcavity electrodes are provided, one 'being coupled to the inputelectrodes and theother being coupled to the output electrodes. y

. The invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing in whichFigure 1 is a cross section of a resonant'cavity diode detector; Figure2'is a plan view of a resonant cavity electrode; Figure 3 is a crosssection of an alternative diode arrangement; Figure 4 is a cross sectionof a triode resonant cavity dis^ charge device; Figure 5 is a curveuseful in explaining the operation of this invention; and Figure 6 is across section of an oscillator or amplifier having two resonant cavityelectrodes. Similar reference numerals refer to similarelementsthroughout the several figures of vthe drawings.

1940; Serial N o. 342,235

(Cl. Z50-27) `ends S'and` I I. invention is not limited to a devicehaving this Referring .t0 Figs. 1 and 2, Within an evacuated glassenvelope- I9 a-member 'I including a resoy nant cavity is mounted.l Thiscavity is bounded by the member 1 which is a metallic electrode in theform of a cylinder having conical reentrant It is to be understoodthatthe particular shape. A cathode electrode I3 is centrally located in oneend 9 of the cavity electrode l, and is capacitively coupled thereto,for example, by means of a conductive sleeve I5 which is connected tothe cavity electrode l. The cathode I3 may be of the oxide-coated typecommonly used in cathode ray tubes and the like. It is heated by aconventional heating element ITI, the

l.leads 2l and 23 to which are brought out through the glass envelope I9in the usual manner. The primary winding of an output transformer 25 isserially connected with a battery 21 between the cathode I3 and anyconvenient point on the cavity electrode 7. Standing waves of ultra highradio frequency energy are set up within the cavity by any convenientmeans; for example, a dipole antenna 29 may be coupled through atransmission line which terminates in a loop 3l within the cavity. K

I have not illustrated specic means for mounting the various elements ofthe tube within the envelope I9, since the mounting is conventional andwill be apparent to those skilled in the art.

In the operation of this embodiment of my invention, the cathode I3emits electrons from its end surface, which electrons followtrajectories throughthe narrow portion of Ithe resonant cavity, asindicated by the dotted lines, and impinge on the opposite end of thecavity electrode. It will be noted that the cavity electrode ismaintained at a positive potential with respect to the cathode.

When modulated radio frequency energy of the proper frequency isreceived by the dipole antenna 29. it is transferred to the resonantcavity, and causes standing waves to be formed therein. The lines ofelectric force of these standing waves are parallel to the electrontrajectories so that the amplitude of the current flowing in theanode-cathode circuit, which includes the output transformer, isdetermined by the amplitude or intensity of the standing waves.Variations in the standing waves due to modulation of the receivedsignal, therefore. produce variations in the anode-cathode current,which, in turn, produce.modulationAfrequency output currents in thesecondary of the output transformer 25.

mum between the surface of the cathode and the opposite end of theresonant cavity electrode which functions as anode. Since the cathode isclosely coupled to the cavity electrode. it forms a part of the highfrequency circuit, and the electric field has a maximum effect on theemission of electrons. 'I'his is an outstanding advantage of theresonant cavity type of tube. Another advantage is'that the capacitybetween the cathode and the effective anode, that is, the portion of thecavity electrode to which the electrons flow, does not act as a shortcircuit to the applied radio frequency, as is the c ase in aconventionalto determine the limiting distance X Ibeyond which the tubewill not operate as a diode, consider a plane cathode and anode, asinFig. 1, separated by a distance X. Then it is known that where I= spacecurrent e=charge of an electron m=mass of an electron V=anode voltage a:cathode area. in square centimeters Substituting known values (1)becomes Thus, both I and t are functions of V and X only, and, if t isgiven, I is determined as a function of either V or X alone; or, if tand I are given, V and X are determined uniquely. If X is taken as thedistance at which the transit time is equal to -a half period, V may bedetermined. I may be obtained from a knowledge of the maximum currentemission produced by a given cathode structure.

Thus, fron. (2) and (4):

.'I X=1.15(l028)tI (5) Assuming I=.02 ampere, and, for an operatingfrequency of 3000 m. c., t=0.17 X -9, so from (5) X=0.17 m. m.=.007 in.

and from (3) V=11.6 volts It is to be -understood that this value is amaxito the resonant cavity. Such an arrangement is illustrated in Fig.3, in which the cathode I3 is mounted in the resonant cavity electrodeas Ibeiore, but which employs a separate anode 33l ca- 10 itself is seento have -a somewhat diilerent shape 2,314,794 The number of lines ofelectric force isamaxivpacitively coupled to the end ofthe resonant.

cavity electrode opposite the cathode. Close coupling is provided sothat both anode 33 and cathode I3 function as an integral part oef theresonant chamber or cavity at high frequencies. An arrangement isillustrated in Fig. 4 in which a grid electrode is mounted on the end ofthe cavity electrode opposite the cathode to .provide triode operation.The resonant cavity electrode from that previously described. Therounded construction illustrated functions in the same manner. Thecathode I3 is centrally mounted in land capacitively coupled to one endof the resonant cavity electrode 1, as before. 'I'hat portion thecathode, forms a part of the high [frequency circuit. The lines ofelectric force which control the electron emission from the cathode areformed as in the preceding case. In the present instance, however,instead of being collected by the cavity electrode. the electrons passthrough the grid and are collected fby the anode which is positionedjust outside the cavity near the grid. As a result, variations in theamplitude of the standingl waves due to modulation of the receivedsignal causes like modulation of the space current rflow to the anode,thus producing modulation frequency currents in the output transformer25.

It will be observed that a battery 3l is con- 5 nected between ground,or cathode, and the cavi- 4.0 trial.v

ty electrode 1 so that the potential of the latter may be made positiveor negative. The anode 33, also, may rbe given a positive or a negativepotential', by suitable adjustment of potentiometer 39, the exact valueof which is best determined by The input transmission line in thisinstance is made in the form of a concentric line Il, al-

though an open line may be employed. Since the line extends into theevacuated region of the tube, it is se'aled'oi by a glass seal 43.4

As in the preceding case, the spacing between cathode and grid and thegrid voltage must be adjusted so that the electron transit time throughthe resonant cavity is less than a half period at the operatingfrequency. In making these calculations, it is necessary to consider theeffect of the d-c field produced by the anode, since the eld will tendto pass through the grid into the cathode grid region. In such a case, Vin the equations given above relating to the diode tube must beconsidered as an effective voltage, and will depend on the gridstructure, anode potential, and spacing of the electrodes.` This eiect,howeverI is well known, since it is also .applicable to conventionaltriode tubs.- A discussion of it is found, for example, in Van Der Bijlsbook, Thermionic Vacuum Tube, paragraph 52 on page 146.

If'the cathode emission -current I of, a tubeof the type illustrated inFig. 1 be plotted as a function ofthe anode voltage V, the resultantcurve will be approximately of the type illustrated in Fig. 5. Velocitymodulation tubes of the type referred to above, in which a constantstream of electrons vis sent into a resonant cavity, operate' along thehorizontal portion of the curve. That is, an increase in the anodevoltage does not cause an increase in the net anode current, but merelyaccelerates a group of electrons. Tubes of the present type, however,operate onan elbow of this curve in amanner similar-to ordinarydiodesandtriodes,since, as ypointed out above, the anode and/orl gridvoltageactually controls the space current.

A radio frequencyamplifier tube isillustrated in Fig. 6, employing tworesonant cavity electrodes 1 and 45 positioned back to back, and

open along their central axis for'the passage 'of electrons.

The input portion of this tubel is substantially identical to the deviceshown in Fig. 4, and need not be described again in detail. Forconvenience, the adjacent surfaces of the two resonant cavities are madefiat so that the transit time, focusing and coupling may be properlyconelectron energy.

If it is desired to use such a tube for the-pros duction of ultra. highfrequency oscillations, 'itisnecessary to provide a feedback pathbetween the output and input cavities. This .is readily accomplished byproviding two aligned openings 55 and 51 in the cavities,- thuspermitting therf lfield in the output resonator to leak back into theinput resonator. A shield 59 is provided, pivoted, for example, at apoint 6I Aand movable between the two apertures, thereby permittingtrolled. As before, the cathode I3 is capacitively 7 cavity byatransmissio'n line 53. The electron current passing through the firstor input cavity ismodulated by the ultra high frequency signals I'applied to the input and set up amplified standingv waves by means vofinduction in the second or output resonant cavity which-are availableinamplified form at the output terminals.

As before, the cathode to first grid vspacing and the electrodepotentials are adjustedand designed to keep the transit time below avhalf period. The purpose of the accelerating grid 41 is to impartgreater velocity to the electrons,

and thus make greater energy available at the output,v since thehigh`ve1ocity electrons set up in the resonant cavity more intensestanding waves than -low velocity electrons. Since the electron velocityis greater between the accelerating and shield grids 41 and 49, .thespacing be` tween them may be increased proportionately for agiventransit time. While grid 49 has been termed a shield grid, it is to beunderstood that all the grids also act as shields to confine the radiofrequency energy within the confines of the two resonant cavities. Theyalso prevent excessive inward fringing of the d-c elds due vInput isapplied to the rst resonant cavity by atra'nsmission line 5|, and outputtaken from the second the amount of feedback to be controlled. Aconvenient method of operating the shield 59may be employed. Forexample, a flexible air-tight bellows 63 is sealed into the glassenvelope I9, or it may be operated by magneticmeans Inoperation as anamplifier, this tube func.

tions as a conventional triode, except 'that resonant cavities areusedas output and input :electrodes. ja 1 `It will be understood that thedescribed discharge devices have particular utility atiultra highfrequencies. Several 'modifications Y have been shown which makepossible: amplication and oscillation, as well as rectification. j-Whllethe invention has been described by meansof several specificembodiments, Iit is to be understood that these` are merely illustrativeof the invention, the scope of which is to be determined y only by thespirit of the/appended claim.

' tron stream to flow through said resonant elec trode comprising meansfor applying a positiveA I claim as my intention:

An ultra high frequency detector comprising a resonant cavity electrodehaving an apertured "1- end, a tubular member forming a projection ofsaid electrode and surroundingsaid aperture, a

'cathode having an electronemissive surface located within'said tubularmember outside of said v y cavity electrode and insulated therefromwhereby Y regions of said cathode-immediately adjacent said emissivesurface are capacitivelycoupled to said tubular member, means forcausing an elecpotential to said electrodewith respecttosaid cathode.means for establishing standing waves of modulated high frequency energyin said cavity electrode. means for varying the electron current -fromsaid cathode in response to the amplitude of said standing waves. andoutput means coupled to said resonant cavity electrode and responr 1sive to said changes in electron current..

3. to the other electrodes'.- The anode 33 is a decel'- eratingelectrode, and it dissipates the remaining;

